Vitrified bonded wheel for electrochemical grinding containing conductive metal and a thermoset polymer filler



Oct. 27, 197. .1. J. AMERO 3,535,832

VITRIFIED BONDED WHEEL FOR ELECTROCHEMICAL GRINDING CONTAINING CONDUCTINE METAL AND A THERMOSET POLYMER FILLER Filed 001:. 1a, 1967 VENTOR JDHN J. XMERU ATTORNEY United States Patent O "ice US. Cl. 51-295 7 Claims ABSTRACT OF THE DISCLOSURE Impregnation of pores of vitrified bonded electrically conductive grinding wheel with organic polymer improves the durability of the wheel without impairing grinding characteristics, forming characteristics, or electrical conductivity.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a grinding wheel for electrochemical grinding and method for making ceramic bonded grinding wheels for use in electrochemical grinding.

Description of prior art Because a vitrified wheel is of ceramic composition, it is fired at relatively high temperatures and is difficult to render conductive. One technique is to impregnate the wheel with a conductive substance. This requires an extensive number of secondary steps, after the initial manufacture of the wheel, such as drying and firing. In addition, the impregnation which coats the walls of the pores in the wheel tends to be non-uniform, giving rise to variations in performance from one wheel to another. The impregnation also tends to be non-uniform within an individual wheel with resulting variations throughout the wheel in internal conductivity.

In electrochemical grinding, resinoid wheels tend to hold their form longer than their vitrified counterparts, but the latter are easier to shape. Special shapes requiring the removal of a significant amount of stock after molding are much easier to prepare as vitrified products.

Prior US patent application, Ser. No. 639,326, filed May 18, 1967, assigned to the assignee of this application, discloses a vitrified bonded grinding wheel in which a special low-melting vitreous glassy bond has, incorporated in it, metallic silver. Such a wheel has excellent electrical properties for electrochemical grinding. Such a wheel, however, suffers from the above-noted inferiority to resinoid wheels in its inability to hold a given shape or form as long as a resinoid type wheel.

SUMMARY OF THE INVENTION The wheel of the present invention retains the advantages of the easy shaping of the vitrified bonded wheel of the prior art, while retaining excellent electrical properties, and, in addition has a shape holding property comparable to the resin bonded wheels.

To fabricate a conductive grinding body in accordance with the invention, finely divided conductive powder is mixed with a ceramic bonding material. Abrasive particles are next wetted and combined with a blend of the bonding material and conductive powder inorder to form an overall mix. The overall mix is then molded into suitable form, such as a grinding wheel. Once molded, the grinding unit is fired at a temperature below the melting point of the conductive powder.

When incorporated into a grinding system, a conduc- Patented Oct. 27, 1970 tive grinding wheel in accordance with the invention is driven by a motor and employed in conjunction with a source of electrical energy which establishes a partial circuit between the grinding wheel and stock being worked. The circuit is completed by introducing an electrolyte between the face of the grinding wheel and the stock.

The fired wheel, after cooling, is then impregnated with a thermosetting resin, such that at least of the available (open) pores in the wheel are filled. The resin is then cured to the thermoset condition. The term thermoset as used herein refers to cross-linked solid polymers which, when subject to a high enough heat, decompose rather than melt. Such thermoset resins may be cured at room temperature, and I do not imply by the use of the term thermoset that the use of an elevated temperature is necessarily employed in the curing process.

The drawing shows a method for impregnating the wheel with resin. A wheel 10 is supported on a plate 11, resting in a container 12. The space between the container and the wheel is sealed by means 13, and the hole, normally formed in a grinding wheel, is closed by plug 14. A connection 15 permits a vacuum to be applied to the container 12. When liquid resin is poured on wheel 10, the vacuum is applied to draw the resin into all the available pores. Upon curing of the resin to the solid thermoset condition, the wheel is ready for use.

DESCRIPTION OF PREFERRED EMBODIMENTS In a preferred embodiment of the invention, the finely conductive powder employed in the bond for the wheel is of silver, of individual particles of average size of about one-half to one micron and present to the extent of approximately 8 percent by volume of the overall mix for an abrasive size of approximately grit. Grit size refers to selecting the size of the grain as specified in Simplified Practice Recommendation 118-50 of the US. Department of Commerce.

In general the appropriate amount of silver depends upon grit size; as the grit size is reduced, the silver content is increased, since there is a greater total area of abrasive to be coated by the powder. With relatively fine abrasive of 120 grit, 9 volume percent silver was found satisfactory. For grit wheels, 4.8% of silver by volume produced an essentially non-conductive wheel unsuitable for electrochemical grinding, While 6% produced a highly conductive wheel suitable for electrochemical grinding. With 46 grit abrasive, only 4% silver was required, while with only 2% silver, the 46 grit wheel was non-conductive. The volume percent is based on the total volume of abrasive, bond (including the silver) and pores.

Silver is the preferred metal to use because it is not subject to deleterious oxidation in the firing of the wheels. Other metals such as platinum are useful but expensive. Copper, unless the particles are protected against oxidation by a silver or other coating, is not desirable. Reducing or nitrogen atmospheres may be deleterious to the ceramic bond, and thus not recommended in most cases even though their use would avoid the oxidation problem in the firing of the wheel. Although the percentage of silver content can be made higher than 9%, there is little to be gained after the necessary level of conductivity has been reached, except for special applications where very high conductivity may be desirable. In such cases, the upper limit of the silver content is restricted only by the physical properties of the fired wheel.

The abrasive may be any of the materials commonly used in the manufacture of grinding wheels, such as silicon carbide. In a tested model of the invention the abrasive was fused alumina and amounted to 48 percent by volume of the overall mix.

The remaining item of the overall mix is the inorganic bonding material. In the case of vitreous bonds, the bonding material is of glass-like composition which fuses at a temperature below the melting point of the conductive metal powder. Useful bonds are those that mature at from 600 to 800 C. Such a composition is principally of clay and glass.

Besides glass frit and clay, the bonding material may include such substances as sodium carbonate, i.e. soda ash, and boric acid. In a tested model of the invention, the bonding material was 50 percent by weight glass frit; 35 percent by weight plastic clay, commonly designated as Imperial ball clay; 7 percent by weight soda ash and 8 percent by weight boric acid.

The process of preparing conductive grinding wheels in accordance with the invention is summarized as follows.

Initially, finely divided conductive powder is intimately blended with an inorganic glassy bond composition having a melting point lower than that of the conductive powder. Next, wetted abrasive particles are combined with the blend of the conductive powder and bonding material to form an overall mix. The overall mix is poured into a mold and pressed, following which firing takes place at a temperature below the melting point of the conductive powder.

The conductive metal powder is obtainable commercially down to an average particle size of approximately one half to one micron.

In the next step of processing, the conductive powder is coarse-mixed with an inorganic bonding material having the characteristics described previously. The conductive powder and bonding material are then intimately blended. In a tested model of the invention, silver powder was intimately blended with a bonding material, primarily of ball clay and lead based glass, by being passed twice through a 100 mesh screen.

In the fourth step of processing, the abrasive is mixed with water until uniformly wet. A test model of the invention added abrasive in the form of fused alumina to a mixer to which water was added until the alumina was uniformly wet.

As mixing continues, the bond-conductive-powder blend is added slowly to the wetted abrasive and mixed until the abrasive appears to be uniformly coated.

For a tested model of the invention, the mix amounted to 48 percent by volume abrasive, 10 percent by volume bonding material, 8 percent by volume silver and the remainder water. In one test the actual quantities were 600 grams abrasive, 113 grams bonding material with a base of lead oxide, 268.2 grams of fine silver powder, and 19.5 cubic centimeters of water.

Once the overall mix is prepared, it is poured into a mold. For a test model of the invention, the mold consisted of conventional constituents, including a mold band, arbor, top plate and bottom plate. The mold pressure was approximately 3 tons per square inch.

After molding is completed, the wheel is dried and fired in air in order to properly fuse the bonding material. In a test model of the invention, the molded wheel was raised from room temperature to a firing temperature of 800 C. at a rate of 100 C. per hour and, once the firing temperature of 800 C. was attained, it was maintained for two hours. It is to be noted that the firing temperature of 800 C. is below the melting point of conductive silver powder and, at the same time, well below the typical firing temperature of 1200 C. for vitrified bonded wheels. The latter temperature, being above the melting point of silver, would result in the formation of a large number of droplets of metallic silver, breaking up the network of conductive paths, and thus materially reducing the suitability of the wheel for electro-chcmical grinding.

A bond having the following composition was prepared:

Material: Weight percent Glass frit 50 Imperial ball clay 35 Soda ash 7 Boric acid 8 The glass frit had the following composition:

Oxide: Weight percent PbO 59.2

SiO 20.0 B 0 14.4 Nazo 6.4

A mixture of silver powder, 120 grit aluminum oxide, 120 grit silicon carbide, and the above bond was made in the following proportions:

Material: Weight percent Alumina 44.4 Silicon carbide 9.1 Glassy bond 14.5 Silver powder 32.0

The mixture was molded in the form of six inch diameter wheels, one-half inch thick, with a one and one-quarter inch hole in the center. The molded wheels were heated to 800 C. at a rate of 100 C. per hour and held at 800 C. for two hours to mature the bond. After cooling, the wheels were impregnated with an epoxy resin to the extent of of the available pores. The liquid resin system was drawn into the wheel by means of a vacuum, by means of apparatus such as shown in the drawing. The liquid is poured on the wheel and a vacuum is drawn. When the liquid appears to have filled the pores, the vacuum is released and the wheel is removed for cure. The epoxy resin of this example was Bakelite resin 3794 (available from Union Carbide) with Bakelite hardener 0803, in a weight ratio of to 37.5, resin to hardener. The cure was effected by holding the wheels at 50 C. for 24 hours. The cure may be elfected at room temperature in several days.

The wheel made according to the preceding example was compared with an identical, but unimpregnated, wheel in the electrolytic grinding of stainless steel. The test conditions were as follows:

Wheel Material wear, removed, Grinding Wheel mlls mllS ratio Amps Volts Untreated 6. 7 50 7. 5 275 10 Resin treated 1.6 52 32. 5 300 The power measured on the wheel spindle was 200 watts, in each case.

The results of this test, under identical operating conditions indicates that the electrical characteristics of the Wheel remain the same with a substantial improvement in wheel life. In spite of the increased durability of the wheel as evidenced by the wheel wear values, forming and dressing characteristics of the resin treated product were not seriously impaired.

Other resins may be used for impregnation such as phenolics, polyesters, polyurethanes, polyimides, and melamines.

Specific resins which, when used to impregnate the conductive vitrified bonded wheels disclosed above, yield grinding wheels of superior durability as shown in tests as described above are as follows:

Phenolic Bakelite BRC2392, a phenol-aldehyde, 100%, baked at 150 C. for hours after impregnation.

Polyester Vibrin 117, a liquid unsaturated polyester, 100 parts, benzoyl peroxide catalyst 3 parts (by weight), baked at 120 C. for 30 minutes, after impregnation. Vibrin is supplied by Marco Chemical Division of W. R. Grace Company. The benzoyl peroxide catalyst consists of 50% benzoyl peroxide in mixed plasticizers and is sold by Reichold Chemicals, Inc. After impregnation, the wheel is baked at 120 C. for 30 minutes to effect the cure.

Polyurethane Adiprene L-167, a liquid reaction product of a diisocyanate and a polyalkylene ether glycol, 100 parts, and Moca, parts, by weight are mixed and impregnated into the wheel. Adiprene is the precursor for a polyurethane elastomer, sold by E. I. du Pont. Moca is also sold by Du Pont and is an activator (4,4 methylene bis(2 chloroaniline). The impregnated wheel is cured by baking at 100 C. for 3 hours.

Melamine aldehyde resin Cymel 301, 100%, an acid catalyzed methoxy methyl melamine one liquid system, supplied by American Cyanamid is impregnated into the wheel and baked at 125 C. for 8 hours.

The ceramic bonded wheels useful in this invention, prior to impregnation, comprise abrasive grains held in place by what are commonly referred to as bonding posts of ceramic glassy bond. The major volume of the total wheel ingredients normally is the abrasive, a typical wheel consisting of 40% or more of abrasive, 10% or more of bond, and the remainder being porosity, most of which is open and available for impregnation. For useful products according to the present invention the unimpregnated wheel should have a porosity of at least and the pores, in the impregnation process must be at least 80% filled in the portion of the wheel which will be employed for grinding, i.e. the annular outer portion of the wheel. Thus in describing the product, although it is essentially pore free since the pores are filled with resin, we will refer to it as a product having filled pores. The completed product is essentially a system of two elements,

the first element consisting of abrasive grains held together by vitreous bond posts, which include an internal continuous conductive phase, and the second element consisting of a continuous phase (intercommunicating) of solid resin.

What is claimed is:

1. A conductive ceramic bonded grinding wheel for electrochemical grinding consisting of abrasive grains bonded together by a ceramic matrix, and including pores within the matrix of abrasive and bond, electrically conductive paths of finely divided metal selected from the group consisting of platinum, silver, and silver coated copper positioned internally of the ceramic matrix, and at least 20% by volume of a thermoset cross-linked solid polymer filling said pores in the portion of said wheel adapted for grinding.

2. A grinding wheel as in claim 1 in which the solid polymer is an epoxy resin.

3. A grinding wheel as in claim 1 in which the polymer is a phenol-aldehyde resin.

4. A grinding wheel as in claim 1 in which the polymer is a cross-linked polyester resin.

5. A grinding wheel as in claim 1 in which the polymer is a polyurethane resin.

6. A grinding wheel as in claim 1 in which the polymer is a melamine-aldehyde resin.

7. A grinding wheel as in claim 1 in which the solid polymer has a liquid precursor.

References Cited UNITED STATES PATENTS 1,403,416 1/ 1922 Katzenstein 51-295 2,243,105 5/ 1941 Kuzmick 51-298 2,421,623 6/ 1947 Kistler 51-295 2,774,108 12/ 1956 Wyllie 51-295 3,062,633 11/ 1962 Coes 51-295 3,216,854 11/ 1965 Halverstadt et al. 51-295 3,310,390 3/1967 Nehru et a1 51-295 3,321,287 5/1967 Hunsberger et al 51-295 3,433,730 3/ 1969 Kennedy 51-298 DONALD J. ARNOLD, Primary Examiner U.S. Cl. X.R. 51-298, 308, 309 

